1. Field of the Invention
The present invention relates generally to a nonpowder process to extrude pellets, more particularly micropellets, to control the size and shape of the pellets as well as retain the chemical integrity of the pellets insuring they are as free as possible of undesirable degradation, premature oxidation, discoloration, premature reaction, expansion, or cross-linking, and to further insure they are of approximately uniform mixture or blend of composition for use in uniaxial, biaxial, and multiaxial minimal shear molding to facilitate enhancement of process temperature, rotation ratios, and rotation rates in view of conventional processes.
2. Description of the Prior Art
Individual processes and equipment for extrusion, pelletizing, drying, conveying, rotational molding, and rock-and-roll molding generally have been known and used in various applications. Separately, chemistries involved in this art have been known. The application of these processes and equipment to the possible chemistries and the extension to enhancement of the minimum shear processes of rotational and rock-and-roll molding has not been readily apparent from the prior art disclosures, and is a focus of the present invention.
Pelletization equipment and its use following extrusion processing has been introduced and/or utilized in applications by the assignee for many years as is exemplified by prior art disclosures including U.S. Pat. Nos. 4,123,207; 4,251,198; 4,500,271; 4,621,996; 4,728,176; 4,888,990; 5,059,103; 5,403,176; 5,624,688; 6,332,765; 6,551,087; 6,793,473; 6,824,371; 6,925,741; 7,033,152; 7,172,397; US Patent Application Publication Nos. 20050220920, 20060165834; German Patents and Applications including DE 32 43 332, DE 37 02 841, DE 87 01 490, DE 196 42 389, DE 196 51 354, DE 296 24 638; World Patent Application Publications WO2006/087179, WO2006/081140, WO2006/087179, and WO2007/064580; and European Patents including EP 1 218 156 and EP 1 582 327. These patents and applications are all owned by the assignee and are included herein by way of reference in their entirety.
Similarly, dryer equipment has been introduced and used in applications following extrusion and pelletization for many years by the assignee as demonstrated in, for example, U.S. Pat. Nos. 3,458,045; 4,218,323; 4,447,325; 4,565,015; 4,896,435; 5,265,347; 5,638,606; 6,138,375; 6,237,244; 6,739,457; 6,807,748; 7,024,794; 7,172,397; US Patent Application Publication No. 20060130353; World Patent Application Publication No. WO2006/069022; German Patents and Applications including DE 19 53 741, DE 28 19 443, DE 43 30 078, DE 93 20 744, DE 197 08 988; and European Patents including EP 1 033 545, EP 1 602 888, EP 1 647 788, EP 1 650 516. These patents and applications are all owned by the assignee and are included herein by way of reference in their entirety.
Minimal shear molding equipment including but not limited to rotation molding and rock-and-roll molding equipment and following from slush molding equipment are similarly disclosed in, for example, U.S. Pat. Nos. 2,629,131; 2,893,057; 3,134,140; 3,315,314; 3,350,745; 3,564,656; 3,676,037; 3,703,348; 3,788,792; 3,810,727; 3,822,980; 3,825,395; 3,841,821; 3,843,285; 3,914,105; 4,022,564; 4,247,279; 4,671,753; 4,705,468; 4,738,815; 4,956,133; 4,956,135; 5,022,838; 5,039,297; 5,188,845; 5,238,379; 5,705,200; 6,030,557; 6,296,792; 6,338,623; 6,511,619; 6,555,037; and 6,604,931; US Patent Application Publication No. 2006/0257518; Canadian Patent No. 2 025 267; Japanese Patent Application No. JP 2005028803; and World Patent Application Publication No. WO 1999/056930. These patents and applications are included herein exemplarily by way of reference and are not limited.
Various pulverulent materials have also been utilized for minimal shear molding including polyethylenes and other polyolefins, vinyl esters and other vinyl polymers, fluoropolymers, polyamides, polyesters, polycarbonates, acrylonitrile-butadiene-styrene (ABS) copolymers, and reactive polymers such as cross-linkable polyethylene (XLPE). Additive packages have been introduced to confer antioxidant stabilization, thermal stabilization, ultraviolet (UV) stabilization, pigmentation, flow modification, processing modification, plasticization, foaming, and cross-linking capabilities by way of example.
Le Roy Payne has introduced equipment and methodology through, for example, U.S. Pat. Nos. 4,671,753; 4,671,991; 4,749,533; 4,909,718; 4,956,133; 4,956,135; 5,011,636; 5,022,838; 5,173,221; 5,188,845; 5,238,379; 5,316,701; 5,503,780; 5,507,632; 5,705,200; 6,030,557; 6,296,792; 6,511,619; 6,555,037; Canadian Patent No. CA 2 025 267; and World Patent Application Publication No. WO 1999/056930 included herein by way of reference that demonstrate the use of pourable polymerizably reactive polyurethane or polyester rotational molding in one or more axes of rotation in optional combination with oscillatory or rock-and-roll processing to generate rotomolded parts. The use of microspheres as additives to at least one of the polymerizable liquid ingredients is demonstrated in U.S. Pat. No. 6,555,037 but is silent on the use of pellets or micropellets as described in the current invention.
Early references for rotational molding composition include use of pellets that were typically more cylindrical and often were ground to provide powders. U.S. Pat. No. 3,514,508 discloses the use of preferably 20 mesh to 50 mesh particles of polyethylene, polypropylene, and polyvinyl chloride in that oven temperatures of 350° F. to 950° F. (approximately 177° C. to 510° C.) are suggested with an inert gas such as nitrogen or carbon dioxide present inside the mold to confer a degree of positive pressure. Larger size particles to two (2) millimeters (mm) are disclosed. An acrylonitrile-butadiene-styrene (ABS) resin containing additives was prepared in sizes ranging from a minus 10 mesh to plus 100 mesh for rotational casting as disclosed in U.S. Pat. No. 3,935,143. Masterblends are disclosed in U.S. Pat. No. 4,508,859 wherein the resin particles and additives are mixed below the softening temperature and this blend upon transfer to an intensive mixer softens the granules sufficiently without fluxing to allow the additive to be absorbed into or onto the granule. Some rounding of the resin granules is anticipated and U.S. Pat. No. 4,508,859 describes use of a 30 or 35 mesh screen to remove unacceptably large granules. U.S. Pat. No. 4,624,818 discloses nylon granules ranging in size from 12 to 35 mesh containing abrasive resistant components preferably ranging in size from 24 mesh to 500 mesh. Similarly, U.S. Pat. No. 4,970,045 discloses the formation of pellets or powder of polyamides and ionomeric copolymers for rotational molding. Controlled pelletization to achieve reproducible particle size distributions, uniform blends, and/or optimized molded article quality or performance was not disclosed.
As described in U.S. Pat. No. 5,525,274, microspheres are prepared wherein polyurethane is extruded and flaked and then mixed with other additives including plasticizers the combination of that is then re-extruded through orifices ranging from 0.007 inches (″) to 0.040″ (approximately 0.178 mm to 1.02 mm). Similarly, polyurethane, polypropylene, polyethylene, polystyrene or polyvinyl chloride is blended with additives including plasticizer in that the resin comprises 50% to 55% of the mixture with extrusion to generate particles dimensionally equivalent as described above. An objective of the U.S. Pat. No. 5,525,274 appears to be to lock the plasticizer into the resin matrix to obtain a cast part of uniform thickness and free of “backside irregularities”. A 50,000 to 150,000 molecular weight range for polyvinyl chloride (PVC) is disclosed as well as that lower molecular weight resins reduce the process temperature and decrease the cycle time.
U.S. Pat. No. 5,525,284 is a divisional patent of U.S. Pat. No. 5,525,274 wherein the highly plasticized microspheres are rotationally cast using a predetermined charge size equal to or greater than the weight of the part being molded or are slush-molded to form thin plastic shells.
U.S. Pat. No. 5,654,102 is a divisional patent of U.S. Pat. Nos. 5,525,274 and 5,525,284 wherein the composition of the microspheres containing plasticizer and limited to 50% to 55% resin are disclosed to be polyurethane, polypropylene, polyethylene, polystyrene, polyvinyl chloride, or epoxies or alloys thereof.
U.S. Pat. Nos. 5,998,030 and 6,410,141 are patents that are continuations-in-part to U.S. Pat. Nos. 5,525,274; 5,525,284; and 5,564,102 and extend the molecular weight range from 2,000 to 150,000. Plasticizer remains a component, although optional, and the resin level in an example therein is within the 50% to 55% range as heretofore described. These patents require that pigment be present and at a level less than 5% by weight.
A pelletizer die for extrusion of micropellets is described in U.S. Pat. No. 5,597,586 that requires a multiplicity of die holes arranged in groups concentrically about the face of the die in that at least one of the groups is composed of die holes of uniform diameter but different than the other groups or in that subgroups within the groups of die holes are uniform in diameter but different than the other subgroups of that group. It has been found that excessive pressure generation and freeze-off effectively blocking at least some of the die holes in practice and this has proven problematic for attempts at implementation.
U.S. Pat. No. 4,238,537 discloses the use of a mixture of pellets and powder both composed of various ethylene vinyl acetate copolymers for rotational molding. The specification discloses that the pellets are typically ⅛ inch by ⅛ inch (approximately 3.2 mm by 3.2 mm) and the powder is ground between 20 mesh and 50 mesh. The disclosed blend is from 50% to 80% pellets and inversely from 50% to 20% powder. Molding is generally done between 500° F. and 625° F. (approximately 260° C. to 329° C.). Complete coalescence of pellet and powder is not requisite according to U.S. Pat. No. 4,238,537 wherein it is disclosed that the pellets retain their particle characteristics at least in part.
European Patent No. EP 0 755 761 and U.S. Pat. No. 5,585,419 disclose the use of micropellets, 0.028 inch to 0.051 inch (approximately 0.7 mm to 1.3 mm) in diameter, in combination with powder, 0.012 inch to 0.035 inch (approximately 0.3 mm to 0.9 mm) and irregularly shaped, of similar composition for use in rotational molding. Similarly U.S. Pat. No. 5,886,068 discloses use of two different micropellet sizes of similar composition wherein the micropellets have a diameter from 0.024 inch to 0.039 inch (approximately 0.6 mm to 1.0 mm) in combination with the larger size ranging from 0.028 inch to 0.051 inch (approximately 0.7 mm to 1.3 mm). Materials disclosed include polyvinyl chloride, polyethylene, polypropylene and Surlyn. Oven temperatures of 600° F. to 800° F. (approximately 316° C. to 427° C.) are maintained wherein the melting temperature is listed at 400° F. to 550° F. (approximately 204° C. to 288° C.). Rotation ratios of 3.75:1 or multiple equivalents are disclosed and large pellet sizes are discouraged as they are disclosed to require longer time to melt and thus fuse after the powder or smaller micropellets have already begun to lay down on the surface of the mold. U.S. Pat. No. 5,886,068 is silent regarding an attempt at separation of layers or use of different materials.
Similarly, U.S. Pat. Nos. 6,433,086 and 6,682,685 describe a rotational molding process using a combination of polyethylene pellets and ground polyolefin powder of the same composition wherein the powder is from 20% to 30% or 20% to 50%, respectively, of the weight of the blend. A single layer is formed from this resin blend that, following U.S. Pat. No. 6,682,685, can include reinforcing fibers to improve structural properties. U.S. Pat. No. 6,833,410 discloses fibers including organic, inorganic, and mineral reinforcing fibers utilized at a loading level of 10% to 80% by weight of a comparable resin pellet to resin powder blend wherein the resin composition is not disclosed.
U.S. Pat. No. 3,368,013 discloses sintering only the edges of various shaped pellets to make multi-vented hollow articles. Similarly, pellets of less than ⅛ inch (approximately 3.2 mm) are sintered according to U.S. Pat. No. 6,030,558 to make porous plastics. The size of the porosity is controlled by the pellet size and sintering is done at temperatures sufficient to only fuse the outside surfaces of the pellets. Note is made of the very narrow particle distribution size range that is particularly beneficial in an application such as selective filtration. For low density polyethylene temperatures are cycled from ambient to 320° F. (approximately 160° C.) and then cooled over a five (5) minute period where particle sizes are 0.031 inch (approximately 0.8 mm). For particle sizes larger than this, cycling as above is followed with heating from two (2) to four (4) minutes at 320° F. (approximately 160° C.) prior to cooling.
WO 2000/035646 demonstrates the use of polyolefin pellets ranging from two carbon to ten carbon monomers, particularly ethylene and propylene, in that the distribution of the particle sizes should meet the criterion of a maximum value of one (1) for a ratio of the difference of the size distribution of an average 0.035 inch (approximately 0.9 mm) particle size and that of an average 0.004 inch (approximately 0.1 mm) particle compared to that of an average 0.020 inch (approximately 0.5 mm) particle size distribution and should contain a water content less than 0.1% by weight according to claim 1. An apparatus requiring at least one additive to be combined with the polyolefin in a mixer, an extruder and pelletizer, a dewatering centrifuge and a fluidized bed drier is described. Rotational molding of the claimed material is also disclosed. WO 2000/035646 describes the residual water content as being no more than 1% by weight. Us Patent Application Publication No. 2005/0228118 is a divisional patent of U.S. Pat. No. 6,894,109 that is itself a divisional patent of U.S. Pat. No. 6,573,314 and only claims minimal or no presence of pinholes in a rotationally molded article. U.S. Pat. No. 6,894,109 reduces this water content to a value less than 0.1% as disclosed in the herein cited WO 2000/035646.
U.S. Pat. No. 6,632,525 discloses the manufacture and use of a plastic particle with a diameter ranging from 0.007 inch to 0.040 inch (approximately 0.18 mm to 1.0 mm) that has a protruding ring on the outer surface of that particle. The ring can be continuous or non-continuous and is produced in an additional heating step following extrusion, pelletization, and cooling of the original pellet sans ring.
Multilayer molded articles have been described in the literature as well. U.S. Pat. No. 3,542,912 discloses the use of a mixture of granules 35 mesh or finer in that the components to form individual layers differ in melting point by at least 10° F. (approximately 5° C.). The individual components are described as immiscible and rotation rates of 20 to 100 rotations per minute (rpm) are described. Oven temperatures are necessarily disclosed at a minimum 20° F. (approximately 11° C.) above the melting point of the highest melting component and typically are maintained at 550° F. (approximately 288° C.). Inner layers, and the higher melting layer, exemplarily include nylon, polycarbonate, acetal, polyphenylene oxide, polysulfones and polyester. The highest melting layer necessarily should also have a higher density than at least one of the other layers. The external, hence lower melting, layer for nylon is suggested to be composed of polyethylene, polypropylene, polyurethane, polystyrene, cellulose acetate, cellulose propionate, cellulose acetate butyrate, acrylonitrile-butadiene-styrene, polyphenylene oxide, polysulfone, styrene-butadiene or polyvinyl chloride. For polycarbonate inner layers only nylon is recommended. Acetal, polyester, or polysulfone inner layers complement polyethylene or polypropylene outer layers. Three layer structures composed of nylon-acetal-polyethylene or polypropylene or alternatively of polycarbonate-nylon-polyethylene or polypropylene.
Sequential formation of layers of similar composition is disclosed in U.S. Pat. No. 3,627,869 where particles ranging from 10 mesh to 400 mesh are deemed acceptable with 16 mesh (0.047″ or approximately 1.2 mm) to 50 mesh (0.012″ or approximately 0.3 mm) considered suitable for rotational molding. Rotation speeds of 10 to 20 rpms are disclosed with oven temperatures given as ranging from 500° F. to 700° F. (approximately 260° C. to 271° C.) or higher with 650° F. (approximately 343° C.) preferred. Polyolefin homopolymers or copolymers were cited in each layer. Similarly, U.S. Pat. No. 4,548,779 generates a first higher melting outer layer then cools the mold and introduces the material to form the inner layer. Preferentially the outer layer is polyamide and the inner layer is polyolefin.
Japan Patent Abstract Publication No. JP 03-000216 discloses a multi-step multi-layer article rotationally molded from powder at approximately 460° F. (approximately 238° C.) wherein each layer is separately charged into the mold. Japan Patent Abstract Publication No. JP 2006-095928 similarly discloses a two-step two-layer process wherein the second material, nylon, is introduced into the mold through a feed nozzle. U.S. Pat. No. 6,982,057 similarly discloses the two-step two-layer process wherein polyethylene, polypropylene or nylon is used as the first layer and the inner layer should be transparent. Oven temperatures of 375° F. to 650° F. (approximately 191° to 343° C.) are described exemplarily of that linear low-density polyethylene was molded at a rotation ratio of 8:2 and 540° F. (approximately 282° C.). The first layer is fashioned to be sufficiently thin to allow removal of an insulating member such that the second flowable component can be poured through the exposed gap. By way of example, the clear second layer now seals over the gap to form a sight line.
Multilayer articles utilizing foamable layers have been made using rotational molding as well. U.S. Pat. No. 3,419,455 discloses a decorative object made from a thermoplastic outer shell and a rigidifying inner can be a layer or can completely fill the inner chamber of the molded article. A skin of reactable resin is introduced into a mold containing a capsule of foaming agent and subjected to rotational molding according to U.S. Pat. No. 3,527,852. Once the skin layer has been formed the foaming agent is controllably released to generate a foamed core article. Similarly, U.S. Pat. No. 3,541,192 introduces a plastisol for formation of the skin layer and then introduces a second foamable or expandable plastisol for controlled generation of the foamed core. U.S. Pat. No. 3,914,361 generates an outer skin and inner foamed core utilizing a plastic powder and a larger expandable particulate simultaneously charged into the sealable mold. Heating from outside melts powder to form the crust and then expands larger particulates to generate the foamed interior.
U.S. Pat. No. 3,936,565 introduces a sandwich structure in that the first layer is charged into the mold and uniformly coated onto the mold surface. The first layer is preferably a cross-linkable polyethylene and the molding temperature is sufficiently controlled to prevent complete cross-linking. On completion of the melt laydown of layer one a second, preferably foamable, layer is charged into the mold and similarly layered without foaming. An optional third layer is also disclosed on the inner surface of layer two and the cross-linking and foaming operations are performed simultaneously to complete the molding process.
U.S. Pat. No. 5,532,282 discloses the simultaneous introduction of a small non-expandable powder, typically 20 to 50 mesh and a considerably larger expandable granule or pellet that is typically 0.040″ to 0.15″ (approximately 1.0 mm to 3.8 mm) in diameter and 0.040″ to 0.125″ (approximately 1.0 mm to 3.2 mm) in length and is specifically disclosed as not needing to be a sphere. Oven temperatures of 580° F. (approximately 304° C.) are cited exemplarily. It is postulated that the smaller particulate melts preferentially allowing the larger pellets to form a second layer and subsequently foam after the first layer is formed. Both a foamed layer and a foamed core are disclosed. The skin-forming layer can be cross-linkable as presented. U.S. Pat. Nos. 5,783,611, 5,830,392, 5,922,778, and 6,038,434 utilize similar methodologies in that improvements of surface properties are achieved by disclosure of carefully controlled chemical formulation and blending. This concept is further extended in U.S. Pat. No. 6,180,203 to include the use of reclaimed material in the larger, higher melting layer that forms the inner skin layer. This additionally provides optional reinforcement rods and a foamable layer between the inner and outer skin of the molded and expanded item.
A mixture of high zero-shear viscosity and low zero-shear viscosity polymeric materials and a third foamable core-forming component are disclosed in U.S. Pat. No. 5,928,584. The patent describes the high viscosity material as a wall-forming barrier that layers against the inner mold surface and forms a barrier to the foamable component preventing its contact with the mold wall. The low viscosity component, that is preferentially cross-linkable, serves as a binding element for the wall-forming layer and the inner foamable core-forming component.
Cross-linkable polyolefins and other reactive resins are well-known in rotational molding and include reactive residual components in the polymeric structure or incorporate “additive packages” that generate bonds, typically by free radicals or other covalent linkages, between portions of proximally located polymers. These additive packages typically include a cross-linking agent, a multifunctional material, and a facilitating agent to enhance the likelihood of cross-linking. Exemplary prior art discussion includes U.S. Pat. Nos. 3,876,613; 3,969,475; 4,029,729; 4,088,714; 4,267,080; 4,414,340; 4,526,916; 4,587,318; 4,678,834; 4,808,364; 4,857,257; 4,900,792; 5,260,381; 5,367,025; 6,528,585; and 6,610,768. Additional examples include US Patent Application Publication Nos. 2005/02024338; 2005/0261430; and 2005/0261443.
Various products are made by rotational molding including toys, chemical storage tanks, water tanks, vehicular fuel tanks, and automotive parts. Decorative effects in polymers have been disclosed including marbling and nacreous effects of salt additives in U.S. Pat. No. 2,330,108 as well as granite-like effects from incorporation of a multiplicity of particulates of comparable or equal density to, isopycnic, and suspended in, typically a transparent or translucent matrix as demonstrated in U.S. Pat. No. 5,304,592.
It is the object of the present invention to provide a cost effective method to prepare reproducible pellets and micropellets by controlled extrusion and pelletization with appropriate drying of rotomolding and extrusional grade materials and rotomolding and extrusional formulations including additives such that minimal or no degradation or undesirable reaction of the components or pellets occurs and to use those pellets to form quality single or multiple layer molded articles utilizing minimal shear rotational and oscillatory processes, including slush-molding, capable of utilizing, in view of the conventional processes, lower processing temperature, reduced processing time, reduced rotation ratios and reduced rotation rates, reducing storage volumes, facilitating in-mold devolatilization, reduction of bridging, ease of transport, safety of operation, reduction of dust hazards, and lower shipment costs by increasing intermediate bulk density such that the nonpowder processes produce a product comparable or superior to the industry standard powder-based products in impact strength, permeability, reproducibility of production, intricacy and trueness of mold detail, uniformity of layer thickness, integrity of layers, bond strength between layers, interlayer uniformity, minimization of bubbles and pinholes and sinks or pockmarks, reduction or elimination of porosity, inner surface quality, structural integrity, gel formation, uniformity of pigmentation and decorative effects.